It has previously been proposed to retort oil shale and other carbonaceous material in a rotating retort by the use of recycled heat carrying solids such as ceramic balls, which are supplied to the rotating retort together with the carbonaceous material. As the very hot balls engage the cooler pre-heated oil shale, the oil shale temperature is raised to a point at which volatile and combustible gases are driven off and collected. Typical prior art patents which disclose processes of this type include U.S. Pat. No. 2,872,386 granted Feb. 3, 1959; No. 3,020,227, granted Feb. 6, 1962; No. 3,265,608, granted Aug. 9, 1966; and No. 3,925,190, granted Dec. 9, 1975.
With regard to the ratio of balls to oil shale which is employed, up to the present time this has been handled on a rather hit or miss basis, without a thorough study of the optimum ratio of the balls or other heat carrying solids to the weight of oil shale, at the particular temperatures which were employed.
With regard to the use of ceramic balls, a persistent operational problem has been the tendency for the balls to crack, chip and break under severe operating conditions. As a result, some operators preferred not to use a difference in temperature between the hot recycled balls and the oil shale feed which exceeded a certain preconceived temperature difference, on the basis that the thermal shock as each ball encountered the cold or cool oil shale would crack the ceramic balls or cause surface chips if too great a temperature difference were present, regardless of the heat transfer coefficient of the system.
Accordingly, it is a principal object of the present invention to determine precisely the factors going into the chipping or breakage of heat carrying solids and to operate retorts on a high efficiency basis, by maximizing heat transfer characteristics of the retorting process through the use of the minimum ratio of the heat carrying solids to oil shale or other carbonaceous material which is being retorted, consistent with maintaining low breakage.